This invention relates to a diagnostic test method for a motor vehicle air/fuel ratio sensor, and more particularly to a method for in-vehicle frequency response testing.
Exhaust gas air/fuel ratio sensors are commonly used for feedback purposes in motor vehicle engine fuel control systems to enable adjustment of engine fuel delivery for achieving a desired intake air/fuel ratio. Consequently, the accuracy of the fuel control under dynamic operating conditions depends to a high degree on the ability of the sensor to quickly respond to changes in the sensed air/fuel ratio. For this reason, it is important to be able to test and verify proper operation of the sensor, both during engine development and periodically during the life of the vehicle.
While formal laboratory testing may be used to determine the frequency response of a sensor, it is impractical to use computationally intensive, formal laboratory analysis methods for field development work and in-use testing. For similar reasons, such analysis methods are also unsuited for on-board diagnostic applications. Accordingly, what is needed is a method of simply and reliably assessing the frequency response of an in-vehicle air/fuel ratio sensor, both for design and development work, and for on-board diagnostic purposes.
The present invention is directed to an improved method of assessing the frequency response of an in-vehicle exhaust gas air/fuel ratio sensor be measuring and analyzing the sensor response to a predetermined perturbation of the fuel delivered to the engine. A first embodiment best suited for design and development work provides both quantitative and qualitative assessment of the sensor response, and a second embodiment best suited for on-board diagnostics provides a qualitative assessment of the sensor response.
According to the first embodiment, the perturbation is achieved by applying fixed biases to the fuel pulse widths of individual engine cylinders to create a rich/lean perturbation in the exhaust gas, and by adjusting the engine throttle to gradually vary the engine speed over a test interval so that the rich/lean perturbation correspondingly varies in frequency. Since the biases are fixed, intake port wall-wetting effects are minimized. According to the second embodiment, the perturbation is achieved by applying an alternating fuel bias multiplier to every engine cylinder, with the engine operating at a fixed speed and load setting that is of interest for diagnostic purposes.
In both embodiments, the output of the air/fuel sensor is band-pass filtered at the frequency of the fuel bias pattern to identify the sensor response, and the sensor response is rectified and low-pass filtered to produce a D.C. measure of the response amplitude. The D.C. measure is then compared with a threshold response to judge if the frequency response of the sensor is within acceptable limits. In the first embodiment, the output of the air/fuel sensor can also be sampled and incrementally processed with a Fast-Fourier-Transform (FFT) technique to identify the response amplitude of the sensor at each of a plurality of frequencies, forming the basis of a Bode plot characterizing the overall response of the sensor.